Add support for C# record structs

[Jand42]

Tests already exist (but marked to skip):

https://github.com/dotnet-websharper/core/blob/d6f1bd43cf0250e31fc1d1c3a1c2244864127599/tests/WebSharper.CSharp.Tests/Records.cs#L55-L62

[ ] Implement ProjectReader changes, only supporting immutable record structs for now (as general mutable struct support is missing)